Some existing communication schemes involve the use of many subcarriers. For example, a wireless communication scheme may employ fifty or more subcarriers, each having a bandwidth of many hundreds kilohertz. Encoded data bits may be interleaved amongst the various subcarriers, and represented as transmission symbols modulated upon the various subcarriers.
Depending upon the physical environment in which the receiver and transmitter are located, some of the subcarriers may exhibit more power than others when received by the receiver. The subcarriers that exhibit a greater power at the receiver are able to carry a greater number of bits per transmission symbol than subcarriers exhibiting a relatively lesser power. Adaptive bit loading schemes make use of this phenomena.
Adaptive bit loading schemes have generally worked as follows. Initially, every subcarrier is assumed to exhibit a sufficient power level to support the most complex form of modulation employed by the communication system. For example, a communication system may employ 64-quadrature amplitude modulation (64-QAM), 16-QAM, quadrature phase shift keying (QPSK), and binary phase shift keying (BPSK). In such a system, each subcarrier is initially assumed to exhibit a sufficient power level to support 64-QAM (which communicates 6 bits per transmission symbol). Therefore, each transmission packet communicated on each subcarrier is encoded using 64-QAM. The transmitter continues to use 64-QAM, until a particular subcarrier is demonstrated to exhibit insufficient power to support such a modulation scheme. Upon such an occurrence, a less sophisticated modulation scheme is used for that particular subcarrier (e.g., 16-QAM, which communicates 4 bits per transmission symbol, may be used). If, at any time, the particular subcarrier is demonstrated to be incapable of supporting 16-QAM, an even less sophisticated modulation scheme may be assigned to the particular subcarrier (e.g., the subcarrier may be assigned to use QPSK, which communicates two bits per transmission symbol).
One shortcoming of the aforementioned scheme is that once a less sophisticated modulation scheme is assigned to a particular subcarrier, the subcarrier will not be re-assigned a more sophisticated scheme for the duration of the connection. This means that, given such a scheme, the data rate may erode over the course of a connection, but cannot regenerate.